Standard clad optical fibers consist of a core having a high refractive index that is clad with a material having a lower refractive index. The mismatch of refractive indices sets the conditions for total internal reflection and gives the fiber its wave guiding properties. Multi-clad fibers have a second layer of cladding, applied over the standard cladding. The second layer has an even lower refractive index, thus permitting total internal reflection at a second boundary. Such internal reflection allows optical fibers to transmit light from one end of the optical fiber to the other. For example, when a light is placed on one end of a fiber, the light is transmitted to the other end of the fiber with a minimal amount of light being lost due to absorption or emission along the fiber's length. Standard optical fibers will only collect and keep light that is somewhat aligned with the axis of the fiber. This means that light that enters the fiber normal to the axis of the fiber is not transmitted to the ends of the fiber.
Scintillating and wave shifting optical fibers can absorb light entering the fiber and re-transmit light of a different wavelength. This property can be exploited when using light emitting diodes (“LEDs”) or other sources to illuminate the scintillating and wave shifting optical fiber. Scintillating and wave shifting optical fibers absorb light in one or more bandwidths and re-emit light at longer wavelengths. For example, a scintillating or wave shifting optical fiber can absorb UV light and emit green light. This absorption and isotropic emission feature improves the efficiency of light collection by the fiber.
Normally, scintillating optical fibers consist of polystyrene-based core, and a polymethyl methacrylate (PMMA) cladding. The scintillating core of the fiber contains a combination of fluorescent dopants, selected to produce the desired scintillation, optical and radiation-resistant characteristics. When a photon passes into the fiber, the energy is absorbed and transferred into light of longer wavelength through excitation of the fluorescent dyes added to the plastic core. The excited light typically uniformly emits in 4π steradians. The portion of the emitted light within the total internal reflection angle of the fiber is then guided down the length of the fiber by total internal reflection from the fiber cladding due to the cladding's lower refractive index. Scintillating optical fibers are used for decoration, when used for novelty or display items, or for radiation detection, when used for industrial purposes. Such fibers are readily available from Poly-Optical Products Inc., Eljen Technology, or from Bicron, a Business Unit of Saint-Gobain Industrial Ceramics, Inc.
Projection display systems, such as heads-up displays (“HUDs”) used in aircraft, use light modulators such as the digital micro-mirror devices (“DMD”), liquid crystal on silicon (“LCOS”) and various liquid crystal displays (“LCDs”). Such modulators, particularly when used in a heads-up display wherein the image created must compete with sunlight, generally need a very high intensity and somewhat collimated light source. One source for such a light is a laser. However, high power lasers are not rugged or eye-safe, can be quite bulky, and are expensive. Light modulators for such displays also need nearly collimated light spread over a relatively wide aperture. Beam-spreading optics are often used to expand a laser beam into a collimated light beam of appropriate size to illuminate a light modulator. The light modulator then controls the transmission, absorption or reflection of the light to projection optics that create a real image on a screen or diffuser.